Unlocked iPhone 3GS with AT&T?

So I already have a iphone 3G and I could re-up with AT&T for 2 years and get a 3GS discounted but I'm travelling soon and think I'll buy an unlocked 3GS. What are the issues if you want to use an unlocked 3GS on AT&T's network? Is activiation required? Will it be recognised? I want to be able to use the phone both on AT&T and any other GSM providers network here and globally.

So what is the buyout fee now? So it makes sense for people to upgrade to the 3gs with the upgrade pricing, unlock, sell old phone and continue to use 3gs with any provider globally. If you pay the buy out free it's then cheaper than having bought the phone from another seller in the first place?

My crystal ball tells me steffi was talking about buying out the remainder of an AT&T contract so that it can be cancelled. This is more correctly known as an Early Termination Fee.

Steffi, the early termination fee only applies if you are within the term of a fixed-term contract.

Apparently you are already an existing AT&T customer using an iPhone 3G. Judging by the fact that you're fully upgrade-eligible, you're probably getting close to the end of your existing 2-year term. In that case, if you manage to obtain an unlocked iPhone 3GS (from some 3rd party, 'cause AT&T won't sell you one that's unlocked, nor will they unlock it for you on request), you can use your existing SIM in it without needing any direct involvement from AT&T. You are not obliged to extend your contract. Once your original contract has expired, you'll be switched to a rolling month-to-month service, which you can terminate at any time without penalty (except perhaps a prorated service fee for the remainder of the final month in your billing cycle).

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